This invention relates to methods for the manufacture of polysiloxanes. In particular, this invention relates to a method for the manufacture of low molecular weight organohydrogen polysiloxanes by ring opening.
Silicones and cured silicone elastomers have a wide variety of commercially important applications, including as greases, adhesives, sealants, coatings, gaskets, encapsulants, potting compounds, and molding materials. Most of these polymers are manufactured from low molecular weight silicone oligomers. Organohydrogen polysiloxanes in particular are used in the synthesis of silicone polymers, and to cross-link a wide variety of resins to form silicone elastomers. Of these, hydride stopped dimethyl siloxanes having the formula EQU (HMe.sub.2 SiO)(SiMe.sub.2 O).sub.x (SiHMe.sub.2)
(M.sup.H D.sub.x M.sup.H), and in particular these hydride stopped dimethyl siloxanes wherein x is 3 form the backbone for durable, hydrophilic silicone elastomers.
A number of methods for the manufacture of M.sup.H D.sub.x M.sup.H have been reported. For example, K. Yoshino and A. Kawamata have reported synthesis by reaction of a cyclic trimer with water, chlorodimethylsilane, SiO.sub.2, and hexane, in JP 02306980 A2 (Dec. 20, 1990). Reaction of the dipotassium salt of hexamethyltrisiloxane diol and chiorodimethylsilane has been reported by K. A. Andrianov et al., in J. Gen. Chem. USSR (Engl. Transl.), Vol. 41, pp. 603-605 (1971).
Ring opening of octamethylcyclotetrasiloxane in the presence of 1,1,3,3-tetramethyldisiloxane with sulfuric acid has been reported by A. A. Zhandov et al. in J. Gen. Chem. USSR (Engl. Transl.), Vol. 43, pp. 1265-1269 (1973); and with an ion exchange resin by S. Schindler, and K. Ruehlmann in Plaste Kautsch., Vol. 25, pp. 384-385 (1978); and with active clays by M. Sakiyaja and R. J. Okawara in Organometal. Chem., Vol. 2, pp. 473-477 (1964).
The redistribution reaction of tetramethyldisiloxane with cyclometallodisiloxane complexes has been reported by W. A. Gustavson, P. S. Epstein, and M. D. Curtis in J. Organometal. Chem., Vol. 238, pp. 87-97 (1982). Ring opening and equilibration of 1,1,2,2-tetramethyldisiloxane (M.sup.H M.sup.H) and octamethylcyclotetrasiloxane (D.sub.4) with an acidic clay such as FILTROL.TM. results in a statistical mixture of products.
Despite the number of routes attempted or known to produce M.sup.H D.sub.x M.sup.H, all have various drawbacks and disadvantages, in that they are either complex, expensive, or difficult to run. Most require a solvent, which increases the cost of the reaction, as well as creates disposal issues. Accordingly, there remains a need in the art for simple, preferably solventless methods for the manufacture of M.sup.H D.sub.x M.sup.H, especially wherein x is three.